The present invention relates to a process for the continuous manufacture of a colourless triethanolamine, which is stable over time and has a high degree of purity, and to the triethanolamine itself which can be obtained by the process of the present invention.
Triethanolamine (TEA) is widely used in industry, especially in fields such as pharmacy and cosmetology, where it has to satisfy a number of increasingly strict requirements.
Among these requirements, the TEA must be colourless. To this criterion is added the stability, particularly thermal stability, of TEA over time. This is because it has been observed that TEA gradually colours over time, without it being possible to identify the products responsible for the coloration, or being possible to explain the mechanisms which result in the coloration. This phenomenon is furthermore frequently mentioned in specialized encyclopaedias and dictionaries such as xe2x80x9cDictionnaire de la Chimie et de ses Applicationsxe2x80x9d [in English xe2x80x9cDictionary of Chemistry and its Applicationsxe2x80x9d] by C. and R. DUVAL, 3rd edition, Technique et Documentation (Paris) (1978), page 1027.
The TEA must also satisfy purity and toxicity requirements. A high degree of purity is often required. Reducing any traces of toxic by-products is also a determining criterion. Among toxic products, certain secondary amines may especially be distinguished, for example diethanolamine (DEA) which, when in contact with nitrated products, forms nitrosamines, which are known as being toxic and carcinogenic.
Surprisingly, it has been observed that the coloration and instability phenomena in TEA over time occur more often when the TEA is pure, and more particularly for a high purity TEA, for example a TEA of purity equal to or greater than 99%, often called in the trade xe2x80x9cTEA 99xe2x80x9d. These phenomena are accentuated by the fact that to keep the TEA in the liquid state it has to be permanently heated above the ambient temperature, e.g. at a temperature comprised between 50 and 70xc2x0 C.
Several solutions have already been proposed hitherto, but none of them seems to be advantageous in terms of cost, ease of processing, purity level, degree of toxicology and, above all, stability, particularly thermal stability, over time.
Most of the proposed solutions generally consist in adding a coloration-inhibiting product to the TEA before, during or after the product has been manufactured. Thus, French Patent Application FR 2 138 902 describes the use of a borate of an alkali or alkaline-earth metal, or of a boric acid alkanolamine ester as coloration inhibitor. The resulting TEA has a relatively high sulphuric ash content.
European Patent Application EP 0 004 015 describes the addition of an active amount of phosphorous or hypophosphorous acid or of their derivatives to the TEA, during or after the preparation of the TEA. No colour index of the TEA was measured after the TEA has undergone a hot-ageing test, and no information was given about residual content of secondary dialkanolamine in the TEA.
Canadian Patent Application CA 2,061,718 describes the addition of relatively great amounts of an amine compound such as diethylenetriamine, triethylenetetramine or tetraethylenepentamine to the TEA. However such an addition neither stops, nor limits the coloration of the TEA from yellow to brown colours, after a hot-ageing test.
U.S. Pat. No. 4,673,762 describes the addition of an alkylene oxide such as ethylene oxide or propylene oxide to the TEA. However these compounds are considered as being very toxic and carcinogenic. No information relating to the residual content of secondary dialkanolamine such as DEA in the TEA was given. In addition, the coloration test of the TEA was carried out at a relatively low temperature e.g. at room temperature or at 49xc2x0 C. (i.e. 120xc2x0 F.).
U.S. Pat. No. 5,292,958 describes a process for eliminating the diethanolamine (DEA) present in triethanolamine (TEA). The process comprises the addition of glyoxal to the TEA containing DEA, in a molar ratio of glyoxal to DEA greater or equal to 1. However this process does not face the coloration and instability phenomena in TEA over time.
U.S. Pat. No. 5,693,866 describes a process for inhibiting colour formation in crude alkanolamines such as crude TEA. The coloration inhibitors are selected from the group consisting of alkali metal hydroxides or sulfites. However the effect of the inhibition of the coloration only relates to crude TEA.
Although these solutions do solve certain aspects of the coloration problems, e.g. only at relatively low temperature or for crude TEA, they create further problems associated with the presence of these inhibitors, which are often used in great amounts or which are undesirable in the end product because of their toxic nature.
Japanese Patent Application JP 62 005 939 describes a heat-treatment of crude TEA in the absence of oxygen, before distillation of the TEA. However the purified TEA obtained after the distillation of the crude TEA thus treated is still sparingly coloured, before any hot-ageing test of the purified TEA.
Another solution proposed in U.S. Pat. No. 4,567,303 consists in preparing the TEA in a reactor in which the corrosion-sensitive part consists of a stainless steel alloy containing no nickel. This solution does not solve all the problems associated with the coloration and above all the instability of TEA over time.
U.S. Pat. No. 3,819,710 describes a process which consists in carrying out a hydrogenation of the ethanolamines in the presence of hydrogen and of a catalyst. The solution proposed is satisfactory neither from an economic or processibility standpoint, nor from a purity or long-term colour standpoint.
Thus, in the field of the manufacture of ethanolamines there is a need to improve the purity of TEA, especially when the latter is manufactured continuously, directly by bringing ammonia into contact with ethylene oxide. In particular, it has become urgent to reduce the toxic nature of TEA, especially by reducing impurities such as secondary amines, for example DEA formed during the preparation of the TEA. For a number of years now there has also been a need to prevent the coloration of TEA during or after its manufacture, and in particular a need to find a process for the continuous manufacture of TEA which makes the latter colourless and stable, especially thermally stable, over time, without involving, in particular, a coloration-inhibiting agent. There is also a need to find a process for improving the stability, especially thermal stability, of TEA over time and for avoiding any coloration, while at the same time improving its purity level and its non-toxic nature.
A simple method has been found which allows the above-mentioned problems to be solved by means of a continuous process for the manufacture, and especially for the purification, of TEA.
The present invention relates first of all to a continuous process for the manufacture of triethanolamine (TEA) comprising, in succession:
(i) a step of synthesizing the TEA by continuously bringing ammonia into contact with ethylene oxide, especially in an aqueous medium, under conditions allowing the formation of a reaction mixture comprising mono- di- and triethanolamines,
(ii) a step of continuously separating the ammonia that has not reacted from the reaction mixture, the ammonia thus separated being preferably recycled in the synthesis step; and
(iii) a step of continuously separating the TEA from the mixture resulting from step (ii), which process is characterized in that, in the last step, a specific mixture of alkanolamines comprising TEA and from 0.5 to 50% by weight of a secondary dialkanolamine is prepared or isolated from the mixture resulting from step (ii), and in that the TEA is separated and isolated with a degree of purity equal to or greater than 99.2%, preferably equal to or greater than 99.5%, and in particular equal to or greater than 99.7% by weight, by continuous distillation of the specific mixture of alkanolamines.
The term xe2x80x9csecondary dialkanolaminexe2x80x9d is understood to mean in general a secondary amine or a dialkanolamine which can have the following formula: 
in which R1 and R2, which are identical or different, represent alkyl radicals each having a primary, secondary or tertiary alcohol functional group and satisfying, for example, the formula xe2x80x94CnH2nOH in which n is an integer ranging from 1 to 12, preferably from 2 to 6 or particularly from 2 to 4.
The secondary dialkanolamines that are preferred are chosen from:
diethanolamine (DEA): HN(CH2CH2OH)2 
diisopropanolamine: HN(CH2CHOHCH3)2 
di-n-propanolamine: HN(CH2CH2CH2OH)2 and
di-n-butanolamine: HN(CH2CH2CH2CH2OH)2 
Diethanolamine (DEA) is most particularly preferred since it is one of the ethanolamines manufactured at the same time as the TEA during the contact between ammonia and ethylene oxide.
Step (iii) of continuously separating the TEA may comprise a prior step eliminating from the mixture resulting from step (ii) the primary ethanolamine formed during the synthesis step (i), in particular monoethanolamine (MEA) and some of the diethanolamine (DEA) by means of a prior distillation, or preferably at least two prior distillations. This or these prior distillations may be carried out in succession and continuously. It (they) may especially be carried out so as to provide the specific mixture with alkanolamines having, according to the present invention, a particular proportion of DEA, that is to say of secondary dialkanolamine, which mixture will be subjected to the continuous distillation making it possible to separate and isolate the desired TEA, that is to say having a high purity and in the form of a colourless product stable over time.
It is also possible, according to another variant, to add to the mixture resulting from this or these prior distillations a secondary dialkanolamine, for example DEA, if the proportion of the latter is too low in the specific mixture of alkanolamines subjected to the continuous distillation in order to separate and isolate the desired TEA. In this case, the amount of secondary dialkanolamine, for example DEA, added is such that, in total, the proportion of secondary dialkanolamine corresponds to that required according to the present invention in the specific mixture of alkanolamines.